Pinewood Biochar in Low-Clinker Cementitious Grouts: Synergistic Effects with Fly Ash and Ladle Furnace Slag under Different Curing Regimes

Incorporating bio-based residues into cementitious materials offers a promising pathway toward sustainable construction. This study investigates the combined effects of biochar (BC, 0–25%), fly ash (FA), and ladle furnace slag (LFS) on the fresh and hardened properties of cementitious grouts. Twenty grout mixes were designed with Portland cement (PC) replacement levels of up to 60%. Mechanical performance and durability-related properties were evaluated after 28 days of water curing and after 210 days of air curing —under sealed and unsealed conditions. Hydration kinetics and microstructural evolution were assessed using isothermal calorimetry, XRD, and TGA. The results indicate that moderate BC incorporation (≤15%) maintained acceptable workability, particularly when combined with FA. Under air curing, higher BC contents resulted in compressive strength (fc) reductions of up to 40% compared to the 100% PC reference. High BC dosage diluted the hydrating matrix and delayed hydration onset, reducing silicate reactions and fc. The kinetic differences were also reflected in the XRD data, showing differences in the intensities of the reflections rather than the type of assemblages. Sealed air curing enhanced the fc of all mixes compared to the unsealed condition (25–80%) and reduced water absorption by 15% —especially in ternary systems— by sustaining hydration and mitigating carbonation. Relative fc of mixes to 100PC reference showed that BC-containing mixes performed relatively better under air curing than water curing, benefiting from the internal curing effect of BC. Flexural strength of BC+LFS mixes reached a comparable value to the reference (≥8 MPa). FA-containing mixes reduced shrinkage (by 22% compared to 100PC) while BC-containing mixes with PC ≥ 60% showed limited carbonation depth (<5 mm). Overall, optimal performance was achieved in ternary blends with BC contents up to 15%, demonstrating a viable strategy to balance mechanical and durability performance, while reducing material embodied carbon by 68% (compared to the reference) in low-carbon cementitious grouts.